Anglerfish

While bioluminescence is uncommon on land, it is surprisingly widespread in the deep ocean. In these dark waters, the vast majority of animals light up as they travel, hunt and mate.

This female anglerfish has her own built-in fishing rod, a modified fin spine topped with a lure that pulses with bacterial light. As she dangles the lure above her gaping jaws, luminous tendrils that look like seaweed trail from her chin.

She belongs to the genus Linophryne, the only group of animals known to use glowing bacteria and also emit their own light.

When a deep-sea anglerfish casts her glowing lure to fish for prey, she may also attract a mate. A male anglerfish is small and weak. When he spots a female, he latches on and becomes a parasite, sponging nutrients from her body for the rest of his life.

Anglerfishes like the one shown here are the only animals known to light up in two ways: with glowing bacteria, and with their own chemicals that make light.

Vanishing Light

Imagine taking a voyage to the bottom of the ocean. As you sink deeper, light from the Sun disappears. A half-mile down (800–1,000 meters), the sea is pitch-black. The only light the human eye detects is bioluminescence.

Sunlight zone: As sunlight enters water, it is scattered and absorbed, so only a thin layer at the top of the ocean is well lit. Most marine plants and animals live here.

Twilight zone: Below 200 meters, sunlight is too dim for photosynthesis—the process plants use to make food. Most animals here are bioluminescent, and many are silvery or transparent. In faint sunlight, these adaptations make them difficult to see.

Midnight zone: Below 1,000 meters, there is no sunlight. Most animals that live here are darkly colored, which helps them stay inconspicuous even when exposed to flashes of living light.